The present invention relates to an etching liquid for etching gold or gold alloy, particularly an etching liquid which can be used for etching gold or gold alloy deposited on a substrate for a semiconductor device or a liquid crystal device.
(I) Gold and gold alloy are widely used as electrode wiring materials for semiconductor devices and liquid crystal display devices. The electrode wiring materials of gold or gold alloy can be processed finely by a wet etching method using an etching liquid.
Gold and gold alloy are conventionally wet-etched with an etching liquid such as an iodine etching liquid consisting of iodine, a salt of iodide and water; a bromine etching liquid consisting of bromine, a salt of bromide and water; and aqua regia (a mixture of nitric acid and hydrochloric acid). The iodine etching liquid is well reactable to gold and gold alloy, resulting in higher etching speed, and is easy to be handled.
A substrate having bump electrodes on which a semiconductor devices and other devices are installed consists of a base substrate of silicon, a primary metal layer formed on the silicon substrate, and a bump electrode(s) (a projecting electrode(s)) of gold or gold alloy formed on the primary metal layer. The primary metal layer has a base layer of Ti/W, Ti/N, Ti/Pt, etc. and a surface layer of gold or gold alloy formed on the base layer. The surface layer is formed so as to improve adhesion of the bump electrode to the base layer.
The substrate with the bump electrodes is produced as follows: firstly a 0.1 to 0.3 μm thick base layer is formed on the silicon substrate; a 0.1 to 1 μm thick surface layer of gold or gold alloy is formed thereon; a resist layer is plated by lithographic technique on the surface of an area of the surface layer other than that on which the bump electrode is to be formed; after that the bump electrodes are formed by depositing a gold or gold alloy layer having a predetermined thickness by a plating method on the surface of the surface layer on which the bump electrodes are to be formed. The bump electrodes project vertically toward the surface of the substrate.
Secondly, the resist layer is removed, so that the surface of the surface layer which has been covered with the resist layer is exposed to the air. The exposed surface layer is removed by wet etching, and the base layer which had been covered with the surface layer is further removed by wet etching. The above process produces the substrate having the silicone base substrate having thereon the fine bump electrodes which project in the vertical direction toward the surface of the base substrate.
The finished substrate further has the above-described metal base layer and the surface layer of gold or gold alloy between the silicon substrate and the bump electrode.
The surface layer should be equally removed by etching it and the bump electrodes should be etched as little as possible during etching the surface layer in order to form the bump electrodes with high precision in size.
However, the metal surface layer may not be removed fully when the surface layer is produced by a sputtering method according to the above-described conventional etching method. This is because as follows.
A rate of etching a sputtered gold or gold alloy layer is generally lower than that of a plated gold or gold alloy layer, which is probably due to the difference in the crystal structure of the layers. Since the surface layer of sputtered gold or gold alloy is etched at a lower etching rate than the bump electrodes of plated gold or gold alloy, the sputtered surface layer may not be removed completely and remain on the substrate as residues when the etching process is conducted such that the bump electrode is etched as little as possible.
The surface layer is especially apt to remain as residues in a portion between column-shaped bump electrodes which are arranged closed to each other, because the portion is in a valley-like shape and the etching liquid is difficult to be diffused to the bottom of the valley-like portion.
When much time is spent in etching the surface layer, the residues will disappear in the entire surface of the substrate including the valley-shaped portion. However, the etching process for a long time etches the bump electrodes in a large amount so that the bump electrodes lack the precision of the form, because the plated bump electrodes are etched at a higher rate than the sputtered surface layers.
Thus the bump electrode is not easy to be formed with high precision by the conventional etching liquid.
(II) Japanese patent publications S51-20976B, S49-123132A, S63-176483A and H6-333911A disclose an etching liquid for etching a minute electrode wiring pattern for semiconductor devices and liquid crystal display devices formed by using photolithographic technique on a substrate, which is added with an alcohol, surfactant and glycerin in order to weaken the surface tension of the etching liquid, whereby its affinity for both the gold and gold alloy and a photoresist film made of a synthetic resin can be improved, resulting in high etching accuracy.
However, none of the Japanese patent publications discloses an etching liquid added with additives such as an alcohol for etching the gold or gold alloy layer from which the photoresist has been removed.
The etching process of etching the gold or gold alloy layer with a mask of the photoresist differs from an etching process of finely processing the same type of metals on the substrate without using the mask wherein a bump electrode and a primary metal layer are etched. It should be noted that teaching of the etching liquid for the former process has not been applied to that for the latter one and no person skilled in the art would have thought to apply it to the latter.